Salts of nitrilotriacetate are useful and well known as detergent builders. They are normally prepared by hydrolysis of nitrilotriacetonitrile. Nitrilotriacetonitrile is also well known, and can be prepared from ammonia or an ammonium salt, formaldehyde, and hydrogen cyanide, or from hexamethylenetetramine, formaldehyde and hydrogen cyanide.
Nitrilotriacetonitrile is hydrolyzed to form a nitrilotriacetate salt according to chemical equations such as the following: EQU N(CH.sub.2 CN).sub.3 +3NaOH+3H.sub.2 O.fwdarw.N(CH.sub.2 COONa).sub.3 +3NH.sub.3
This reaction can also be accomplished using hydroxides of other alkali metals. The product can be isolated as crystals of the monohydrate, or can be sold as a solution, usually about 40%, in water.
The nitrilotriacetate salt must be substantially colorless to be useful commercially as a detergent builder. It is preferred that a 40% solution of the nitrilotriacetate salt in water have an APHA color no greater than 100, or more preferably no greater than 50. The standard method of meeting the color requirements of the product is utilization of excess hydroxide. Excess hydroxide is known to reduce color formation in the nitrilotriacetate product. But, the excess hydroxide must be removed from the product, such as by a separation of the crystallized product from the mother liquor containing the excess hydroxide. This mother liquor must either be disposed of or recycled. Because of the extra steps associated with the use of excess hydroxide, a process to produce low color nitrilotriacetate salts using stoichiometric hydroxide would be an advancement in the art.
Commercially useful nitrilotriacetate salts, whether sold as crystalline material, or as a solution, must be of high purity, and must not contain appreciable levels of undesirable impurities, such as cyanide. Additionally, a commercial process should produce this product in as simple and straightforward a manner as possible, and with minimal capital and energy requirements. This invention meets these objectives in the following manner.